The present invention relates to a filter arrangement and a method for a telecommunication device.
In telecommunication systems of today it has become more and more common that information of different types are transmitted on a common transmission line in different frequency bands. An example of such a system is ADSL (Asymmetric Digital Subscriber Line) which enables parallel transmission of conventional speech telephony (POTS) and wideband services, such as video or data, over conventional copper twin cable networks. This technique makes it possible to use existing copper twin cable networks more efficiently, which is of utmost interest since it is very costly and time-consuming to replace the copper cable with, for example, an optical fibre having higher capacity. There is also a high feed embodiment of ADSL termed VADSL or VDSL (Very High Speed ADSL).
Since POTS traffic only uses the frequency band up to 4 kHz, a large part of the frequency band available in PSTN (Public Switched Telephone Network) is often unused. In an ADSL system a larger frequency range such as for example 25-1000 kHz is used for wideband services of about 1,5-6 Mbit/s. The information transmitted in the higher frequency range is termed ADSL traffic or ADSL signals. When the telecommunication traffic reaches a receiver of a subscriber or in a central office of a service provider, the POTS and ADSL traffic is separated, often with the use of a so called splitter filter. The splitter filter comprises a low pass filter between the transmission line and the telephone of the subscriber/the line card of the central office and a high pass filter between the transmission line and the ADSL receiver of the subscriber or in the central office.
In order to obtain optimal transmission characteristics and in order to avoid echoing, it is advantageous if the filter used on the telephone side of the ADSL system have an impedance similar to the impedance of the transmission line, which is described by John Cook and Phil Sheppard in xe2x80x9cADSL and VADSL Splitter Design and Telephony Performancexe2x80x9d, IEEE Journal on selected areas in communications, Volume 13, No. 9, December 1995. Since the copper twin cable has a complex impedance it is desired that also the impedance of the filter is complex. It is however not possible to manufacture lossless passive filters having a complex impedance, but by imbedding the passive filter between impedance converters, so called GICs (General Impedance Converter) the impedance of the filter appears to be complex outside the impedance converters. This technique is described in the paper by John Cook and Phil Sheppard cited above and in the European patent EP0742972. GICs can be implemented in several different ways, for example such as is described in the document cited above or as is described in the Swedish patent application SE 9800240-5.
A problem with the technique described above is that filters are very expensive. In an ADSL system two splitter filters per subscriber are required, consequently if the cost for one filter can be reduced large savings can be achieved. For this reason one strives to manufacture filters having as few and as inexpensive components as possible. This must be balanced against the fact that the filters can be made more effective the more components they have. In the selection of filter a trade-off between costs and quality must thus be made.
As stated above the costs for a splitter filter constitute a large portion of the total costs in a telecommunication system where information is transmitted in different frequency bands, such as in an ADSL or VADSL system. It is therefore desirable to manufacture splitter filters comprising as few and as inexpensive components as possible. Inductors and components comprising inductors, such as filter coils and transformers, are expensive components, which it is desirable to have as few as possible of in a splitter filter. However, there is a problem in that in a filter having few filter coils there are fewer degrees of freedom for adjusting the efficiency of the filter than in a filter having many filter coils. A filter having many filter coils can thus provide better characteristics, for example better stop band attenuation or a smoother pass band, than one having few coils.
According to a first aspect the present invention deals with the problem of how a filter arrangement can be implemented having fewer inductors in order to obtain a lower cost while at the same time the characteristics of the filter arrangement is not worsened and how a method for frequency filtering and impedance conversion can require as few inductors as possible without reducing the quality.
According to a second aspect the present invention deals with the problem how a filter arrangement can be given better characteristics without increasing the total number of inductors in the filter arrangement and how a method for frequency filtering and impedance conversion can be improved without using more inductors.
The object of the present invention is thus to obtain an efficient and worthwhile filter and to obtain an efficient and worthwhile method for frequency filtering and impedance conversion.
The invention solves the problem according to the first aspect by means of integrating the filter and an impedance converter connected to the filter. This is obtained in that an already present filter coil is provided with an additional winding and is used both as a filter component and as a component in the impedance converter. The filter coil having the additional winding can replace a transformer in the impedance converter, which results in saving one inductor.
The invention solves the problem according to the second aspect by means of integrating the filter and an impedance converter connected to the filter. This is obtained by means of replacing a transformer in the impedance converter with a component having two windings. This component is impedance converting and at the same time one of the windings is used as a filter coil. Hereby the filter arrangement is given an additional active filter component without increasing the total number of inductors. The additional active filter component can be used for improving the characteristics of the filter arrangement.
Thus, the invention according to both the first and the second aspect provides an efficient filter arrangement and a method comprising an integrated unit, which acts both as an impedance converter and as a filter.
The invention according to the first aspect is related to the case when a filter having at least one filter coil is flanked by at least one impedance converter (GIC), which comprises a transformer. The impedance converter senses an electrical magnitude in or in connection with the filter and adds a serial voltage, which is dependant upon the sensed electrical magnitude, to the circuit. According to the invention the filter coil can be provided with an additional winding and replace the transformer, whereby the filter characteristics and the added GIC serial voltage are maintained. This is obtained in that the additional winding of the filter coil is supplied with a current I so that an electromotive force exe2x80x2 is induced in the filter coil. The current I is chosen so that the induced electromotive force exe2x80x2 is equal to the GIC serial voltage. The filter coil having the additional winding is at the same time used as a frequency filtering component in the filter and provides the same filter characteristics as the original filter coil. The invention can thus, according to the first aspect, contribute to saving one transformer per GIC compared to the prior art.
The invention according to the second aspect is related to the case when a splitter filter is flanked by at least one impedance converter (GIC), which comprises a transformer. The impedance converter senses an electrical magnitude in or in connection with the filter and adds a serial voltage, which is dependent upon the sensed electrical magnitude, to the circuit. According to the invention, the transformer can be replaced by a component having two windings, where the first winding is used as an active filter component and which generates the GIC voltage e by means of feeding the current I to the second winding, so that an electromotive force exe2x80x2 is induced in the first winding. The current I is chosen so that the induced electromotive force exe2x80x2 is equal to the GIC voltage e. The first winding providing an active filter can be used for improving the characteristics of the filter. Thus, the invention according to the second aspect can contribute to the implementation of a more effective filter arrangement and a more effective method, which use the same number of inductors as a corresponding filter arrangement and a corresponding method according to prior art. This is obtained by means of increasing the number of inductors acting as active filter components without increasing the total number of inductors in the filter arrangement.
An advantage of the invention according to the first aspect is that a more inexpensive filter arrangement and a more inexpensive method for separating information transmitted in different frequency bands in a telecommunication system can be made without deteriorating the filter characteristics. A filter arrangement according to the invention becomes more inexpensive than previously known solutions, having the same filter characteristics, since it can be made using fewer expensive transformers. An additional advantage of the invention according to the first aspect is that the filter arrangement according to the invention occupies less space than the previously known solutions since transformers are relatively large components.
An advantage of the invention according to the second aspect is that one can implement a filter arrangement and a method for separating information which is transmitted in different frequency bands in a telecommunication system, where the filter arrangement has better filter characteristics than previously known solutions having the same number of inductors and where the method provides better filter characteristics than previously known methods using the same number of inductors.
Yet another advantage of the invention according to both the first and the second aspect is that a filter arrangement and a method for separating information transmitted in different frequency bands in a telecommunication system can be implemented, where the inductors used are utilized more efficiently than in corresponding filter arrangements and methods according to the prior art.